1. Field of the Invention
The present invention relates to a method and a device for purifying an exhaust gas of an internal combustion engine. More specifically, the invention relates to a method and a device which are capable of efficiently purifying hydrocarbon in the exhaust gas released from the engine during a starting up period.
2. Description of the Related Art
An exhaust gas purification device utilizing a three-way reducing and oxidizing catalyst (hereinafter referred to as a "three-way catalyst") is commonly used for purifying HC, CO and NO.sub.x from the exhaust gas of an internal combustion engine. However, a three-way catalyst is capable of purifying HC, CO and NO.sub.x only when the temperature of the catalyst becomes higher than a activating temperature (for example, about 300.degree. C.). Therefore, when the temperature of the three-way catalyst is low, such as in the starting up period of the engine, HC and CO in the exhaust gas pass through the three-way catalyst without being purified. Further, since the amount of fuel supplied to the engine is increased during the starting operation in order to facilitate the engine start, a relatively large amount of HC (unburned hydrocarbons) is contained in the exhaust gas in the period immediately after the engine has started compared to that during a normal operation. Also, since the amount of the fuel supplied to the engine during a starting operation is increased as the temperature of the engine is lower, a large amount of HC is released during a cold starting of the engine. This large amount of HC is released to atmosphere without being purified by the three-way catalyst since the three-way catalyst does not reach the activating temperature during cold starting of the engine. Therefore, it is necessary to purify HC in the exhaust gas during a starting operation of the engine in order to reduce air pollution due to engine exhaust gases.
To solve this problem, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-79319 discloses an exhaust gas purifying device which utilizes an HC adsorbent for adsorbing and temporarily storing hydrocarbons released from the engine during starting operation. The device in the '319 publication disposes an HC adsorbent in the exhaust gas passage of the engine. The HC adsorbent adsorbs hydrocarbons in the exhaust gas when the temperature of the adsorbent is lower than a releasing temperature determined by the material of the adsorbent. Further, the HC adsorbent releases the adsorbed hydrocarbons when the temperature becomes higher than the releasing temperature. The device in the '319 publication further includes an electrically heated catalytic converter disposed in the exhaust gas passage downstream of the HC adsorbent. The electrically heated catalytic converter (hereinafter referred to as "an EHC") is a catalytic converter provided with an electric heater which is capable of heating the catalyst in a short time. By energizing the heater at the start of the engine, the catalyst is heated to the activating temperature rapidly. According to the device in the '319 publication, a relatively large amount of the hydrocarbons contained in the low temperature exhaust gas during the starting up period of the engine is adsorbed and temporarily stored in the HC adsorbent. Therefore, during the starting up period of the engine, i.e., when the temperature of the catalyst is lower than the activating temperature, hydrocarbons in the exhaust gas do not pass through the catalytic converter, and hydrocarbons are not released into the atmosphere. When the catalyst in the converter reaches the activating temperature, the catalytic converter starts to oxidize the hydrocarbons in the exhaust gas. Therefore, even if the HC adsorbent is saturated with the adsorbed hydrocarbons after the EHC reaches the activating temperature, hydrocarbons which are not adsorbed by the HC adsorbent and flow into the EHC are purified by the EHC.
However, when the temperature of the HC adsorbent reaches the releasing temperature due to a temperature rise after the engine has started, the hydrocarbons adsorbed and stored in the HC adsorbent are released in a short time, and the concentration of hydrocarbons in the exhaust gas flowing into the EHC largely increases. In this case, the air-fuel ratio of the exhaust gas flowing into the EHC becomes very low due to the hydrocarbons released from the HC adsorbent. In this condition, the EHC cannot oxidize hydrocarbons due to a shortage of oxygen in the exhaust gas. Therefore, the device in the '319 publication disposes a temperature sensor for detecting the temperature of the HC adsorbent bed, and supplies secondary air to the exhaust gas passage between the HC adsorbent and the EHC when the temperature of the HC adsorbent becomes higher than the releasing temperature. Further, the device adjusts the amount of secondary air by a feedback control based on an oxygen concentration sensor disposed in the exhaust gas passage at the inlet of the EHC in order to supply an exact amount of secondary air required for purifying (oxidizing) hydrocarbons in the exhaust gas to the EHC.
Namely, in the device disclosed in the '319 publication, relatively large amounts of hydrocarbons released from the engine during the starting up period are stored in the HC adsorbent, and when the hydrocarbons stored in the HC adsorbent are released due to a temperature rise of the exhaust gas, secondary air is supplied to EHC in order to purify the released hydrocarbons on the EHC.
However, in the device in the '319 publication, a temperature sensor for detecting the HC adsorbent bed temperature is required to determine the timing at which the releasing of hydrocarbons occurs. Further, an oxygen concentration sensor and a feedback control system of secondary air are required to supply a proper amount of air to the EHC because, if an excess amount of air is supplied to the EHC, the fuel consumption of the engine will increase due to the power consumption of an air source such as an air pump in the secondary air system and, if the amount of air is not sufficient, hydrocarbons are not sufficiently purified on the EHC. Therefore, the device in the '319 publication is complicated and the manufacturing cost is relatively high.
Further, since the oxygen concentration in air is only about 20%, a relatively large amount of secondary air must be supplied to the EHC in order to purify all the hydrocarbons released from the HC adsorbent. Therefore, in the device in the '319 publication, the EHC, even after it reaches the activating temperature, may be cooled by a large amount of secondary air to a temperature below the activating temperature and may loose its activity. This lowers the ability of the EHC to purify hydrocarbons in the exhaust gas.